Tyrosine-specific Protein Kinases (PTKs) are a family of enzymes which catalyze the transfer of the terminal phosphate of adenosine triphosphate (ATP) to tyrosine residues in protein substrates [for review see: Hunter, T; Protein modification: phosphorylation on tyrosine residue; Curr Opin Cell Biol 1989; 1:1168-1181]. The first members of this class of enzymes to be identified were PTKs encoded by viral oncogenes, which were capable of cell transformation (ie. pp6Ov-src and pp98v-fps). Later it was shown that there were normal cellular counterparts of these viral gene products (ie. pp60C-src and pp98c-fps). Since that discovery, a large number of genes encoding PTKs have been identified [for review see Hunter, T; Protein kinase classification; Methods Enzymol 1991; 200:3-37]. These include growth factor receptor PTKs such as the insulin and epidermal growth factor receptors, as well as non-receptor PTKs such as ZAP-70 and Lck. Although the molecular details have yet to be fully elucidated, PTK-mediated phosphorylation of tyrosine residues on protein substrates leads to the transduction of intracellular signals that regulate a variety of intracellular processes such as growth, transport, motility, and senescence. Many disease states are dependent on these cellular functions. Therefore, inhibitors of tyrosine kinases are useful for the prevention and chemotherapy of disease states that are dependent on these enzymes.
For example, tyrosine kinase inhibitors are useful for inhibiting T-cell activation and thus they are useful as immunosuppressive agents for the prevention or treatment of graft rejection following transplant surgery and for the prevention or treatment of autoimmune diseases such as rheumatoid arthritis and psoriasis. Graft rejection following transplant surgery is a common occurrence which arises when foreign antigens are recognized by the host immune system. In an effort to protect itself from the foreign tissue, the host immune system is then activated to release an arsenal of antibodies, soluble lymphokines, and cytotoxic lymphocytes which attack the foreign tissue, resulting in complications which often end in graft rejection. Similarly, a breakdown in self-tolerance can result in immune system attacks against the body's own tissues. These attacks can lead to autoimmune and chronic inflammatory diseases. Since T cells are the key regulators of these immune system attacks, inhibitors of T cell activation are useful therapeutic agents.
Currently the leading medicinal agent for the prevention or treatment of graft rejection is Cyclosporin A, approved by the United States Food and Drug Administration in 1983. Cyclosporin A is extremely effective at preventing transplant rejection and is efficacious in the treatment of autoimmune disorders such psoriasis, rheumatoid arthritis, inflammatory bowel disease, and type I diabetes. It work by forming complexes with a specific protein which can then inhibit the catalytic activity of calcineurin, a phosphatase that plays a key role in transducing signals from the T cell receptor (TcR) to the nucleus. However, calcineurin is ubiquitously expressed and is involved in many other signal transduction pathways. As a result, Cyclosporin A suffers drawbacks in that it can cause kidney failure, liver damage and ulcers; which in many cases can be very severe. Consequently, Cyclosporin A has a very narrow therapeutic index and is rarely used to treat chronic autoimmune diseases. Safer drugs which are more selective in their ability to affect the immune response system and which have fewer side effects are constantly being pursued. Thus, there is a continuing need and a continuing search in this field of art for alternative therapies. The Src-family protein tyrosine kinase, Lck, is upstream of calcineurin in the TcR-mediated signaling cascade. Lek is expressed almost exclusively in T cells and its catalytic activity is required for T cell signal transduction [for review see: Anderson S J, Levin S D, Perlmutter, R M; Involvement of the protein tyrosine kinase p56lck in T cell signaling and thymocyte development; Adv Immunol 1994; 56:151-178]. Thus, a potent Lck-selective kinase inhibitor would make a promising drug candidate.
Lck is one of 8 known members of the human Src-family of protein tyrosine kinases. The other members are Src, Fyn, Lyn, Fgr, Hck, Blk, and Yes. As a consequence of alternative mRNA splicing, Fyn exists as two distinct gene products, Fyn(T) and Fyn(B), that differ at their ATP binding sites. All Src-family kinases contain an N-terminal myristoylation site followed by a unique domain characteristic of each individual kinase, an SH3 domain that binds proline-rich sequences, an SH2 domain that binds phosphotyrosine-containing sequences, a linker region, a catalytic domain, and a C-terminal tail containing an inhibitory tyrosine. The activity of Src-family kinases is tightly regulated by phosphorylation. Two kinases, Csk and Ctk, can down-modulate the activity of Src-family kinases by phosphorylation of the inhibitory tyrosine. This C-terminal phosphotyrosine can then bind to the SH2 domain via an intramolecular interaction. In this closed state, the SH3 domain binds to the linker region, which then adopts a conformation that impinges upon the kinase domain and blocks catalytic activity. Dephosphorylation of the C-terminal phosphotyrosine by intracellular phosphatases such as CD45 and SHP-1 can partially activate Src-family kinases. In this open state, Src-family kinases can be fully activated by intermolecular autophosphorylation at a conserved tyrosine within the activation loop.
Src-family kinases display a variety of tissue-specific expression patterns. Src, Fyn(B), Yes, and Lyn are found in a broad range of tissues with especially high levels of expression in neuronal and hematopoietic cells. The expression of these particular Src-family kinases overlap to a great extent, however no cell types have been found that express all four of them. Expression of Lck, Fyn(T), Fgr, Hck, and Blk is restricted to cells of the hematopoietic lineage. In general, myeloid cells co-express Hck, Fgr, and Lyn; immature B cells co-express Hck, Lyn, and Blk; and mature B cells co-express Hck, Lyn, Blk, Fgr, and Fyn(T). T cells predominantly express Lck and Fyn(T). Lck is also expressed in NK cells.
A complex cascade of biochemical events mediates signal transduction in T cells [for review see: Chan A C, Desai D M, Weiss A; The role of protein tyrosine kinases and protein tyrosine phosphatases in T cell antigen receptor signal transduction; Annu Rev Immunol 1994;12:555-5921. While many proteins involved in this signaling cascade have been identified, the molecular details of this process are just beginning to be unraveled. The antigen-specific .alpha./.beta. TcR heterodimer is noncovalently associated with CD3-.epsilon., -.delta. and .zeta. polypeptide chains. In the current paradigm of T cell activation, stimulation of the TcR by MHC/peptide complexes on the surface of antigen presenting cells (APCs) leads to the rapid activation of Lck. Activated Lck then phosphorylates CD3 and .zeta. proteins on tyrosine residues within conserved motifs known as ITAMs (Immunoreceptor-associated Tyrosine-based Activation Motifs). Another protein tyrosine kinase, ZAP-70, is recruited to the TcR complex via association of its tandem pair of SH2 domains to doubly phosphorylated ITAMs. Lck, in turn, activates TcR-associated ZAP-70 by phosphorylation of tyrosine 493 in the ZAP-70 activation loop. Activated ZAP-70 goes on to phosphorylate a variety of downstream adapter molecules such as LAT, SLP-76, and HS1. Lck can also phosphorylate additional protein substrates in activated T cells. One important substrate is Vav, a guanine nucleotide exchange protein that is regulated by Lck phosphorylation. Activated Vav mediates GDP release by Rac/Rho family members which, in turn, leads to the reorganization of the actin cytoskeleton, an event that is necessary for T cell activation. In addition to TcR recognition of MHC/peptide complexes on the surface of APCs, there are many co-receptor pairs that are important in T cell-APC interactions. Of note are CD4 and CD8, which are associated with Lck and bind to nonpolymoiphic regions of MHC Class II and Class I molecules, respectively. Other co-receptor pairs include CD28/B7, CTLA-4/B7, LFA-2/LFA-3, LFA-1/ICAM, CD40/CD40L, SLAM/SLAM, and etc./etc. This vast array of cell-cell molecular interactions stabilizes T cell/APC conjugates and initiates additional intracellular signaling cascades. Signals derived from co-receptor engagement are integrated with signals derived from stimulation of the TcR to determine the magnitude and the quality of the T cell response.
Genetic data clearly validate Lck as an excellent therapeutic target. Mice in whom Lck expression was perturbed by either genetic deletion or by overexpression of a catalytically inactive version of Lck exhibited an early block in T cell development. The small number of mature T cells in the periphery of Lck-deficient mice were inefficient at transducing signals from the TcR and could not mediate a vigorous response to antigenic challenge. NK cells from Lck deficient mice appeared to function normally. No functional defects outside of the immune system were noted in these animals. In addition there is a report in the literature of a human patient with low levels of Lck expression due to an inability to properly splice Lck mRNA [see: Goldman F D, Ballas Z K, Schutte B C, Kemp J, Hollenback C, Noraz N, Taylor N.; Defective expression of p56lck in an infant with severe combined Immunodeficiency; J Clin Invest 1998; 102:421-429]. This patient presented with Severe Combined Immunodeficiency Syndrome (SCID). Again, no other phenotypic disturbances outside of this immune system disorder were noted. These results strongly suggest that Lck inhibitors would be effective in suppressing T cell mediated immune responses without causing mechanism-based toxicity.